Flexible joints that provide restoring forces may be known, for example, from U.S. Pat. No. 3,990,116 to Fixel et al. and U.S. Pat. No. 3,266,059 to Stelle. A problem with conventional flexible joints may be the difficulty of integrating them into new products. The conventional joints that are easily integrated into new products may not, in general, provide restoring forces. In addition, the conventional joints may not include an overstrain sensor.